Copper infrastructure in the access network is ubiquitous and has served our voice communication needs well for over a century. As data connectivity needs have grown in the last twenty years, several technologies have been introduced to exploit this existing copper network and expand its usefulness. These include narrow-band modems, various versions of DSL, ISDN, DDS and T1/E1 technologies.
The Internet era has introduced new demands on the access network. Bandwidth demand is growing at 25% or more per year for the average enterprise. But this demand is constrained by the “service gap”—the void in services and pricing between T1 and T3 service. Creative solutions that address this gap in a cost-effective way can increase penetration (total lines in service), margins, and revenue.
T1/E1 service can now be delivered through a single copper pair using an HDSL2 or G.shdsl modem. Data services at similar or higher speeds can be delivered using ADSL. Newer versions of this technology, such as VDSL, offer speeds of more than 50 Mb/s when operating in an asymmetrical mode (more bandwidth in one direction than in the other). This is remarkable, given the fact that the copper plant was originally designed having only voice services in mind.
The difficulty has been in delivering high symmetrical speeds to all, or even the majority of customers, in a cost-effective manner. For example, at 10 Mb/s or greater speeds, VDSL has significant reach limitations and can only serve a small percentage of the customer base. Before it can be widely adopted, a major and costly re-engineering of the outside plant environment is needed to reduce the average loop length.
The vast majority of customers lie within the Customer Service Area (CSA), defined as 12,000 feet of 24 AWG cable or 9,000 feet of 26 AWG cable. At this range, symmetric 1.5-2.0 Mb/s service is close to the highest bit rate service that a single copper pair can deliver reliably. For speeds greater than this, fiber-based services are most commonly deployed today.
Unfortunately, fiber is not ubiquitous in the access network. Current estimates are that less than 7% of all businesses can be reached by fiber. This is expected to increase to just over 10% by 2006. Recent announcements of expansion of the fiber network by US telecommunications carriers talk about bringing fiber to an additional 1 million US customers per year for the next several years, which represents about 0.7% of the total customer base per year.
Construction of new fiber in the access network is typically focused on high-density environments such as multi-tenant office buildings in large cities. But in the Internet era, the demand for high-speed connectivity is widespread geographically.
Fiber is expensive to deploy. Construction costs can be significant as can the cost of the equipment itself. Customers located “off net” often cannot justify the up-front construction costs of fiber, not to mention the dramatically higher monthly charges for fiber-based service.
This has created the “service gap”. From an enterprise perspective, the jump from traditional copper to fiber-based service is a large one. As customer demand for bandwidth continues to grow, this service gap will become increasingly apparent and problematic.
There are many limitations of the copper loop plant as a communications medium. Copper twisted pairs are usually of small gauge, resulting in significant signal power reduction over long distances. Despite this attenuation, however, the capacity of a copper twisted pair at CSA range would be well above typical T1 rates if interference and noise could be suppressed. Unfortunately, copper pairs are typically not shielded and incur substantial ingress noise and interference from other lines. This is known as crosstalk.
Often, just a single service activated in the same binder can result in a 50% reduction in the capacity of a copper pair compared to the case where no other services are present in the binder. Similarly, ingress noise from RF sources, radio stations, electric motors, etc., can result in significant performance degradation for the lines in the affected binder. The present invention significantly mitigates those limiting factors and provides dramatic bitrate improvements through the use of physical-layer coordination, commonly referred to as “vectoring”.
Existing approaches to multiline transmission utilize “bonding” of multiple copper pairs. “Bonding” refers to the combination of multiple copper pairs at the digital layer. In this approach, the incoming datastream is partitioned at the transmitter end into multiple datastreams, each of which is transmitted over one of the individual copper pairs without regard to the other copper pairs. Then, these multiple datastreams are reassembled at the receiver end into a single datastream. The operations of partitioning and reassembling the datastream have the undesirable side effect of adding latency to the transmission. Moreover, since the physical layer of the individual copper pairs is left unchanged, the total bitrate is less than or equal to the sum of the bitrates of the individual pairs.
Crosstalk in a multiline system takes two distinct forms, which are addressed by different techniques. Crosstalk originating from the multiline system's own transmitters is referred to as “in-domain” crosstalk, while crosstalk originating from other sources is referred to as “out-of-domain” crosstalk. Besides crosstalk, multiline systems suffer from spectral leakage, poor time domain equalizer (TEQ) performance, poor windowing, and poor analog front end (AFE) designs.